Method for liquefaction of a stream rich in hydrocarbons

ABSTRACT

A method for liquefying a hydrocarbon-rich stream is disclosed. In an embodiment, the hydrocarbon-rich stream is liquefied in a heat exchanger countercurrent to a three component refrigerant mixture. The refrigerant mixture is compressed in a two stage compressor. The refrigerant mixture is separated into a higher boiling fraction and a lower boiling fraction. A fluid fraction is recovered from a partial stream of the lower boiling fraction. The fluid fraction is supercooled and expanded to a pressure of the higher boiling fraction and the fluid fraction is provided to a compressor stage to which the higher boiling fraction is taken.

This application claims the priority of International Application No. PCT/EP2006/001804, filed Feb. 28, 2006, and German Patent Document No. 10 2005 010 055.4, filed Mar. 4, 2005, the disclosures of which are expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a process for liquefying a hydrocarbon-rich stream, specifically a natural gas stream.

Natural gas liquefaction plants are laid out either as what are known as LNG baseload plants—plants for liquefying natural gas to provide natural gas as primary energy—or as what are known as peak shaving plants—plants for liquefying natural gas to meet peak demands.

Larger LNG plants are usually operated with refrigeration circuits which consist of hydrocarbon mixtures. These mixture circuits are more energy-efficient than expander circuits and allow relatively low specific energy consumption.

From German Patent Document No. DE-A 102 09 799 a process for liquefying a hydrocarbon-rich stream, specifically a natural gas stream, is known in accordance with which the liquefaction of the hydrocarbon-rich stream takes place in the heat exchange countercurrent to a two-component refrigerant mixture stream; the one component is a part of the hydrocarbon-rich stream to be liquefied, while the other component is a heavy hydrocarbon, preferably propane or propylene. Before the cooling and the expansion to provide refrigeration of these components, the refrigerant mixture is separated into a higher boiling and a lower boiling refrigerant fraction.

A disadvantage of the procedure described in DE-A 102 09 799 is that providing two refrigerant components can result in relatively large temperature differences in the heat exchangers. These temperature differences in turn require correspondingly high compressor performance.

A similar process for liquefying a hydrocarbon-rich stream is known from U.S. Pat. No. 6,347,531. In this process, the low-pressure refrigerant is inducted cold through the circulating compressor. These cold-inducting compressors have the disadvantage that in operation, in particular during start-up and shut-down, they are more complicated to operate than compressors not inducting cold. Furthermore, in the liquefaction process described in U.S. Pat. No. 6,347,531 it is disadvantageous that the refrigerant is partially liquefied at an intermediate pressure, which results in greater expense for equipment.

The object of the present invention is to specify a generic process for liquefying a hydrocarbon-rich stream, specifically of a natural gas stream, which avoids the disadvantages of the known processes and in addition allows a lower specific energy requirement to be realized.

To achieve this object, a generic process for liquefying a hydrocarbon-rich stream is proposed, wherein:

the liquefaction of the hydrocarbon-rich stream takes place in the heat exchange countercurrent to a three- or multi-component refrigerant mixture,

one of the components is a part of the hydrocarbon-rich stream to be liquefied,

one of the components is propane, propylene or a C₄ hydrocarbon,

one of the components is C₂H₄ or C₂H₆,

the compression of the refrigerant mixture stream takes place by means of an at least two-stage compression,

before the cooling and the expansion of the refrigerant mixture to provide refrigeration, the refrigerant mixture is separated into a higher boiling and a lower boiling refrigerant fraction, and

the higher boiling and the lower boiling refrigerant fractions, after their expansion to provide refrigeration are taken at different pressures to compression.

Surprisingly, it has been shown that the specific expenditure of energy for liquefaction by means of the process in accordance with the invention can be reduced by approximately 30%. Furthermore, the temperature differences within the heat exchanger or heat exchangers can be reduced significantly. The result is that transient operation is easier to control.

Additional advantageous embodiments of the process in accordance with the invention for liquefying a hydrocarbon-rich stream are:

the refrigerant mixture is a three-component refrigerant mixture,

the refrigerant fractions are cooled separately, expanded separately to provide refrigeration and heated separately countercurrent to the hydrocarbon-rich stream to be liquefied,

a further component of the refrigerant mixture is nitrogen,

compression of the refrigerant mixture stream takes place by means of an at least two-stage compression and the higher boiling refrigerant fraction is admixed to the lower boiling refrigerant fraction at an intermediate pressure level,

at least one C₄ to C₆ hydrocarbon is used as further component(s) of the refrigerant mixture; the use of additional refrigerant components makes sense in particular at greater liquefaction outputs above 10 t/h, and

at least one partial stream of the lower boiling refrigerant fractions is partially condensed and the liquid fraction obtained thereby is supercooled and expanded.

The process in accordance with the invention and additional embodiments of the invention are to be explained in what follows using the embodiment shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE illustrates an embodiment of the invention for liquefying a hydrocarbon-rich stream.

DETAILED DESCRIPTION OF THE DRAWING

In accordance with the procedure shown in the drawing, a dry, pre-treated hydrocarbon-rich stream, for example natural gas, is taken to the liquefaction process in accordance with the invention through line X and liquefied in heat exchanger E and supercooled if required. The hydrocarbon-rich stream is, as an example, at a pressure of between 10 and 60 bar. The liquefied and, if necessary supercooled, hydrocarbon-rich stream is then taken through line X′ for further use. Not shown in the drawing is a separation, which may have to be provided, of undesirable components, for example higher hydrocarbons. For this, reference is made to the appropriate explanations in the aforementioned DE-A 102 09 799.

The cooling and liquefaction of the hydrocarbon-rich stream X, X′ takes place in accordance with the invention in the heat exchange countercurrent to a three or more component refrigerant mixture stream where one of the components is part of the hydrocarbon-rich stream to be liquefied—preferably methane—one of the components is propane, propylene or a C₄ hydrocarbon and one of the components is C₂H₄ or C₂H₆.

The corresponding refrigeration circuit preferably has a two-stage compressor unit, consisting of the compressor stages C1 and C2. An air or water cooler—not shown in the drawing—is located in series with each compressor stage. The refrigeration circuit further has a high-pressure extractor D. Providing only one high-pressure separator D reduces the operating cost of the process in accordance with the invention substantially—compared with the known refrigerant mixture circuits.

In the separator D, the refrigerant mixture is separated into a lower boiling and a higher boiling fraction. The lower boiling fraction is removed from the separator D through line 2, cooled in the heat exchanger E, condensed and supercooled and then expanded at the cold end of the heat exchanger E in expansion valve b, providing refrigeration. The expanded fraction is again taken to the heat exchanger E through line 3, evaporated and superheated therein countercurrent to process streams to be cooled and then taken to the first compressor stage C1 through line 4.

Following compression and cooling not shown in the drawing, the compressed lower boiling fraction is taken to the second compressor stage C2 through line 8—the admixture of the higher boiling fraction will be discussed in more detail in what follows—and compressed to the desired circulation pressure which is, for example, between 20 and 60 bar. A heat exchanger as cooler not shown in the drawing is also located in series with the second compressor stage C2. The refrigerant mixture cooled and partially condensed in the cooler is taken back to the separator D through line 1.

A higher boiling liquid fraction is drawn off from the bottom of the separator D through line 5, cooled in the heat exchanger E and then expanded in expansion valve a to the desired intermediate pressure, providing refrigeration. Then this fraction is taken back to the heat exchanger E through line 6, evaporated and superheated therein countercurrent to process streams to be cooled and then taken through line 7 to the compressor unit ahead of its second compressor stage C2.

In accordance with an advantageous embodiment of the liquefaction process in accordance with the invention, at least one partial stream 9 of the lower boiling refrigerant fraction 2 can be drawn off from the heat exchanger following cooling and partial condensation through the broken line 9, and taken to (“cold”) separator D′ indicated by broken lines. The gaseous fraction drawn off at the head of the separator D′ through line 10 indicated by broken lines, is again returned to the heat exchanger E, supercooled and expanded for the purpose of providing the peak cold in valve b required for the liquefaction process.

The liquid fraction drawn off from the bottom of the separator D′ through the broken line 11 is supercooled in the heat exchanger E, expanded in valve c providing refrigeration, taken to the heat exchanger E through line 12 and admixed to the refrigerant fraction in line 3.

Additional “cold separators” can be provided in addition to this separator D′. They result in an improvement of the specific energy requirement of the liquefaction process in accordance with the invention, but they make sense only in larger liquefaction plants because of the additional expense required for equipment.

The higher boiling fractions recovered in the separator D′ and any additional “cold separators” are preferably supercooled, expanded to the pressure of the (first) higher boiling fraction and taken to the compressor stage to which the (first) higher boiling fraction is also taken. This embodiment of the process in accordance with the invention is indicated in the drawing by the dotted line 13. Depending on the temperature profile in the heat exchanger E, admixture to the low-pressure refrigerant stream in line sections 3 and 4 also makes sense.

In accordance with an advantageous embodiment of the inventive process, the liquefaction of the hydrocarbon-rich stream takes place countercurrent to the refrigerant mixture in plate heat exchangers. Because of the process management in accordance with the invention, process management can be realized in a single plate heat exchanger in liquefaction plants having a liquefaction capacity of up to 10 to 15 t/h.

The process in accordance with the invention to liquefy a hydrocarbon-rich stream, specifically a natural gas stream, avoids all the disadvantages of the prior art cited at the beginning. 

1-7. (canceled)
 8. A process for liquefying a hydrocarbon-rich stream, specifically a natural gas stream, wherein: liquefaction of the hydrocarbon-rich stream takes place in heat exchange countercurrent to a three or more component refrigerant mixture; one of the components is a part of the hydrocarbon-rich stream to be liquefied; one of the components is propane, propylene or a C₄ hydrocarbon; one of the components is C₂H₄ or C₂H₆; compression of the refrigerant mixture is carried out by means of an at least two-stage compression; before cooling and expansion of the refrigerant mixture to provide refrigeration, separation of the refrigerant mixture into a higher boiling and a lower boiling refrigerant fraction takes place; the higher boiling and the lower boiling refrigerant fractions following their expansion to provide refrigeration are taken at a hot end of the heat exchange at different pressures for compression; and at least one partial stream of the lower boiling refrigerant fraction is partially condensed and a fluid fraction recovered is supercooled and expanded; wherein the fluid fraction recovered is expanded to a pressure of the higher boiling fraction and taken to a compressor stage to which the higher boiling fraction is also taken.
 9. The process according to claim 8, wherein the refrigerant mixture is a three-component refrigerant mixture.
 10. The process according to claim 8, wherein the refrigerant fractions are cooled separately, expanded separately to provide refrigeration and heated separately countercurrent to the hydrocarbon-rich stream to be liquefied.
 11. The process according to claim 8, wherein a further component of the refrigerant mixture is nitrogen.
 12. The process according to claim 8, wherein at least one C₄ to C₆ hydrocarbon is used as an additional component of the refrigerant mixture.
 13. The process according to claim 8, wherein a second fluid fraction is recovered and supercooled, is expanded to a pressure of the lower boiling fraction and taken to a compressor stage to which the lower boiling fraction is also taken.
 14. The process according to claim 8, wherein the liquefaction of the hydrocarbon-rich stream takes place countercurrent to the refrigerant mixture in a plate heat exchanger.
 15. The process according to claim 14, wherein the plate heat exchanger is a single plate heat exchanger.
 16. A method for liquefying a hydrocarbon-rich stream, comprising the steps of: liquefying the hydrocarbon-rich stream in a heat exchanger countercurrent to a three component refrigerant mixture; compressing the refrigerant mixture in a two stage compressor; separating the refrigerant mixture into a higher boiling fraction and a lower boiling fraction; recovering a fluid fraction from a partial stream of the lower boiling fraction; supercooling and expanding the fluid fraction to a pressure of the higher boiling fraction; and providing the fluid fraction to a compressor stage to which the higher boiling fraction is taken.
 17. The method according to claim 16, wherein the lower boiling fraction is taken to a first stage of the compressor.
 18. The method according to claim 17, wherein the higher boiling fraction is taken to a second stage of the compressor.
 19. The method according to claim 16, wherein the fluid fraction is supercooled in the heat exchanger.
 20. The method according to claim 16, wherein the fluid fraction is expanded in an expansion valve external to the heat exchanger.
 21. The method according to claim 16, wherein the fluid fraction is recovered in a separator.
 22. The method according to claim 16, wherein the partial stream of the lower boiling fraction is taken from the lower boiling fraction in the heat exchanger.
 23. The method according to claim 16, wherein after the step of expanding the fluid fraction, the fluid fraction is provided to the heat exchanger.
 24. The method according to claim 21, wherein the fluid fraction is drawn off from a bottom of the separator.
 25. The method according to claim 24, wherein a gaseous fraction of the partial stream of the lower boiling fraction is drawn off at a head of the separator.
 26. The method according to claim 24, wherein the gaseous fraction is provided to the heat exchanger.
 27. The method according to claim 26, wherein the gaseous fraction is provided to the lower boiling fraction. 